Field of the Invention
The present invention is related to nanofibers, methods and devices for depositing the nanofibers on curved surfaces, and application of articles formed from the deposited nanofibers.
Description of the Related Art
Fabrication of Filter Media and Filters
The filtration industry has traditionally manufactured particulate air filters using conventional medium such as glass, cotton or polymer fibers made provided as rolled goods. The fibrous media may be made by non-woven processes such as wet laid paper, melt blown-spinning or woven yarn. The material is then transported to equipment where the media is cut, pleated, supported, glued into filter frames, and tested for leaks. Various measures of the properties of the rolled goods include appropriate weight per unit area, porosity, etc.
The porous filter media may be pleated or bonded into bags to increase the area of the media within individual filter units to reduce pressure drop. Often screens and other supports are added to prevent collapse of the media from the force of air flowing through the filter unit as dust is collected. Depending on the intended use of the filter, the filter may be tested with an appropriate challenge aerosol at a rated or standard airflow rate for pressure drop and particle collection efficiency, (e. g., ASHRAE 52.2, MIL-STD-282, IEST RP-CC 007.1, NIOSH APRS-STP-0051-00, and NIOSH APRS-0057-00 may be used to test the filters)
Theoretically, a reduction of the diameter of the fibers in a filter has the potential of causing an improvement of the filter system performance. For high efficiency filtration, fiberglass wet-laid papers are widely used having fiber diameters in the 200 nm to 5000 nm size range with the fiber sizes intentionally blended for both durability and filtration performance.
One technique for producing a smaller fiber diameter, and hence a potential for generating improved filtration media, is electrospinning of polymers to make submicron and nanofibers. Electrospinning as currently practiced uses a constant voltage to drive the spinning process defined herein as static field electrospinning. The following patents describe conventional ways to fabricate nanofiber containing filters for various applications: U.S. Pat. Nos. 7,008,465; 6,994,742; 6,974,490; 6,955,775; 6,924,028; 6,875,256; 6,875,249; 6,800,117; 6,746,517; 6,743,273; 6,740,142; 6,716,274; and 6,673,136; the entire contents of each of these patents are incorporated in entirety herein by reference.
The typical approach for making goods from nanofibers is to deposit them onto a pliable substrate such as a woven or nonwoven textile. The substrate supports the nanofibers and makes it possible to handle and transport them. The deposition of the nanofibers is typically conducted either in a planner or cylindrical fashion. For planner geometries a roll-to-roll process can be used as well as a single sheet of some finite dimension; these process are similar to that observed in conventional nonwoven and paper making industries. For cylindrical geometries a rotating cylinder is used to collect the fibers. The nonwoven media is then used either as a cylinder (e.g. a tube) or is cut and used as a sheet. A review of various techniques for fabricating nanofiber containing goods is given by Teo and Ramakrishna (2006) entitled “A review on electrospinning design and nanofibre assemblies,” the entire contents of which are incorporated herein by reference.
Conventional layered nanofiber filters made from nanofibers deposited on conventional porous filter media have inherent limitations. The support media of these filters is usually pliable enough to allow pleating or manipulation during the assembly step. Such a pliable substrate media may flex or stretch from the air pressure drop force and may break or debond the nanofibers. The support layer of conventional media may contribute substantially to the pressure drop of the whole structure.
Filtration Metrics and Challenges to Nanofiber Media:
In one metric, filtration performance is described by the particle collection efficiency and filter pressure drop (i.e. resistance to air flow). Filtration efficiency (Eff) for a particular filter of piece of filter media is a function of the particle size and the air flow rate. Filter pressure drop (ΔP) is a function of the air flow rate. For high efficiency filters and filter media (Eff>95%) it is often more convenient to report fractional particle penetration (Pt). For example a high efficiency particulate air (HEPA) filter media with Eff=99.97% for 0.3 μm particles has fractional penetration Pt of 3×10−4. A convenient metric for comparing filter media is the filter figure of merit (FoM), which considers particle collection efficiency per pressure drop. There are several ways to calculate FoM but here we use FoM=−Log(Pt)/ΔP, where ΔP is expressed in kPa. FoM is determined for a particular particle size and filter face velocity. The filter face velocity is the ratio of the air flow rate to the area of the filter media. In conventional HEP A media, the FoM is 12±2 kPa−1 for 0.3 μm particles and a face velocity of 5.3 cm/s.